Superconducting cable with the phase conductors connected at the ends

ABSTRACT

A superconducting cable for high power with at least one phase includes a superconducting core wherein a plurality of elements are housed, which are structurally independent and magnetically uncoupled, each of which includes—for each phase—a couple of phase and neutral coaxial conductors, each formed by at least a layer of superconducting material, electrically insulated from one another by interposition of a dielectric material. As a result of the distribution of the superconducting material into several coaxial conductive elements, the cable allows to transmit high current amounts in conditions of superconductivity, while using a high-temperature superconducting material sensitive to the magnetic field. The conductive elements are connected at the ends to yield a mean exploitation efficiency of 100%.

DESCRIPTION

In a general aspect, the present invention relates to a cable to be usedto transmit current in condition of so-called superconductivity, i.e.,in conditions of almost null electric resistance.

More particularly, the invention relates to a superconducting cable forhigh power having at least one phase, including a superconducting corecomprising a phase conductor and a neutral conductor, external to theformer and coaxial to the same, each including at least a layer ofsuperconducting material, said coaxial conductors being electricallyinsulated from one another by interposition of a dielectric material, aswell as means for cooling said core at a temperature not higher than thecritical temperature of said superconducting material.

In the following description and the subsequent claims, the term: cablefor high power, indicates a cable to be used for transmitting currentquantities generally exceeding 5,000 A, such that the induced magneticfield starts to reduce the value of the maximum current densityachievable in superconductivity conditions.

In the following description and the subsequent claims, the term:superconducting material, indicates a material, such as for instancespecial niobium-titanium alloys or ceramics based on mixed oxides ofcopper, barium and yttrium, or of bismuth, lead, strontium, calcium,copper, thallium and mercury, comprising a superconducting phase havinga substantially null resistivity under a given temperature, defined ascritical temperature or T_(c).

The term: superconducting conductor, or, shortly, conductor, indicatesin the following any element capable of transmitting electric current insuperconductivity condition, such as for instance a layer ofsuperconducting material supported by a tubular core, or tapes ofsuperconducting material wound on a supporting core.

As is known, in the field of energy transmission, one of the problemsmore difficult to solve is that of increasing as much as possible boththe current to be transmitted in superconductivity conditions and thetemperature at which the transmission takes place.

Even though the so-called “high-temperature” superconducting materialsare available today, which can transmit currents at temperatures of theorder of 70-77° K (about −203/−196° C.), a reduction in the currenttransmission capacity by said material is noticed when the inducedmagnetic field increases.

See on the matter, for instance, T. Nakahara “Review of Japanese R&D onSuperconductivity”, Sumitomo Electric Technical Review, Nr. 35, January1993.

In superconductivity conditions, the sensitivity of superconductingmaterials to the effects of the induced magnetic field is ever moremarked the greater is the working temperature of the superconductingcore of the cable (i.e., the superconducting materials with the highestcritical temperature are more sensitive to the effects of the magneticfield), so that in practice high-temperature superconducting materialsdo not allow for transmission currents higher than some KA, without anunacceptable increase in the quantity of superconducting materials to beused, and, along therewith, of the associated costs.

In the case of the so-called coaxial cables, whose configuration issuitable for transmission of high loads, the induced magnetic field, thetransmitted current and the diameter of the conductor are tied by thefollowing relation:

B=(μ_(o)×I)/(π×D)

wherein:

B=magnetic field on the surface of the conductor;

I=transmitted current;

μ_(o)=magnetic permeability;

D=diameter of the conductor.

(As is known, the values of B and I are to be understood as directcurrent actual values, or as alternate current effective values).

On the basis of this relation, it ensues that each increase in thetransmitted current brings about a proportional increase in the inducedmagnetic field, which in turn limits, to a greater or smaller extent,the maximum current density obtainable in superconductivity conditionsor technical critical current density, “J_(e)”, defined as the ratiobetween the critical current and the total cross section of the layer ofsuperconducting material.

More particularly, it has been noticed that the critical current densitydrastically decreases—sometimes up to two orders of magnitude—startingfrom a threshold value of the magnetic field, lower than the criticalfield above which the superconductivity is substantially compromised;indicatively, such value varies from 0.1 to 20 mT according to thesuperconducting material used and to the working temperature. In thisregard, reference is made to, for instance, IEEE TRANSACTIONS ON APPLIEDSUPERCONDUCTIVITY, vol. 5, nr. 2, June 1995, pp. 949-952.

The attempts made to keep the critical current density at acceptablevalues based on an increase in the conductor diameter, have until nowfailed, due to both the practical difficulty of making, transporting andinstalling a large diameter cable, and the high costs necessary to coolthe superconducting core, as the thermal dissipations are proportionalto the diameter of the insulating layer that surrounds the core of thesuperconductor.

Therefore, in view of these difficulties of technological nature, in thefield of coaxial cables the art has been substantially restricted toeither transmitting the desired high current quantities by means ofsuitable metal or ceramic materials at the temperature of 4° K, at whichthe aforementioned phenomena are less marked, or accepting an other thanoptimum exploitation of the superconducting material at the maximumtemperature (65°-90° K) compatible with current transmission insuperconductivity conditions.

In the first case, one has to face the high costs associated with theneed of cooling the superconducting core at a very low temperature,while in the second case it is necessary to use a very high quantity ofsuperconducting material.

According to the invention, it has now been found that the problem oftransmitting within a coaxial cable having at least one phase highcurrent quantities at the maximum working temperature of thesuperconducting materials available today (65°-90°K, determined by theusable materials and cooling fluids) can be solved by splitting up foreach phase the superconducting material within the cable into aplurality of “n” elements, structurally independent and magneticallyuncoupled, each of which comprises a couple of phase and neutral coaxialconductors, insulated from one another, and transmits a fraction “I/n”of the total current.

According to the invention, in fact, it has been found that with suchdistribution of the superconducting material it is possible to:

a) reduce the size of the cable, with the same use conditions of thesuperconducting material, with the ensuing easiness of construction,transport and installation of the cable;

b) use, with the same quantity of superconducting material, the samequantity of electric insulating material of conventional cables;

c) limit, with the same quantity of superconducting material, the sizeof the thermal insulation layers (cryostat) which surround thesuperconducting core of the cable, with an advantageous reduction inthermal losses; and

d) have superconducting elements which, in case of need, canindependently supply different loads.

Preferably, the phase and neutral coaxial conductors of each of saidelements comprise a plurality of superimposed tapes of superconductingmaterial, wound on a tubular cylindrical support, for instance made ofmetal or insulating material.

In order to reduce as much as possible the possible mechanical stressesin their inside, the tapes of superconducting material are wound on saidsupport according to windup angles—either constant or variable from tapeto tape and within each individual tape—of 10° to 60°.

Alternatively, the phase and neutral coaxial conductors of each of saidelements may comprise a plurality of layers of superconducting material,superimposed and laid on the tubular cylindrical support.

According to the invention, the maximum number of coaxial conductiveelements is determined by the minimum diameter of such elementscompatible with the winding deformations of the tapes made ofsuperconducting material, or compatible with the critical tensiledeformation of the superconducting material chosen.

Preferably, the diameter of the phase conductor of each of said elementsvaries from 25 to 40 mm.

According to the invention, the superconducting core of the cable iscooled at temperatures not higher than 65°-90° K, dvantageously usingso-called high-temperature superconducting materials and liquid nitrogenas cooling fluid.

Among these high-temperature superconducting materials, use mayadvantageously be made of those known in the art by the initials BSCCOhaving the formula:

Bi_(α)Pb_(β)Sr_(γ)Ca_(δ)Cu_(ε)O_(x)  (I)

wherein:

α is a number from 1.4 to 2.0; β is a number from 0 to 0.6; γ is anumber from 0 to 2.5; δ is a number from 0 to 2.5; ε is a number from1.0 to 4.0; and x is the stoichiometric value corresponding to thedifferent oxides present.

According to the invention, particularly preferred are mixed oxides ofthe following ideal general formula:

(BiPb)₂Sr₂Ca_(n−1)Cu_(n)O_(x)

wherein n is a whole number from 1 to 3 and x is the stoichiometricvalue corresponding to the different oxides present.

Among them, particularly advantageous results have been obtained withthe mixed oxide known as BSCCO-2223 (i.e., in which n=3), or withsuitable mixtures of mixed oxides of the aforementioned metals, in suchratios as to obtain a mean stoichiometry of the mixture corresponding tothat of the BSCCO-2223 oxide.

In another aspect, the present invention relates to a method fortransmitting a current quantity higher than a prefixed value within asuperconducting cable having at least one phase, which method ischaracterized in that said current is split up, for each phase, among aplurality of magnetically uncoupled conductive elements of a coaxialtype, the number of such conductive elements being such that the currentfraction carried in each of them is lower than a value which determinesa superficial current density in each of the conductive elementscorresponding to a magnetic field capable of generating a conductivityreduction of a superconducting material used, where superficial currentdensity is defined as I/(π×D) and has the units A/m, where I is thetransmitted current and D is the diameter of the conductor.

In a particular embodiment, such current is a multiphase alternatecurrent, and said conductive elements among which the current is splitup, carry a single phase of said current.

In a preferred embodiment of the method, said predetermine quantity ofcurrent is at least equal to 5,000 A. In the method according to theinvention, and if liquid nitrogen is used as cooling fluid, the magneticfield capable of generating a conductivity reduction of thesuperconducting material used is lower than 200 mT, preferably lowerthan 100 mT and more preferably lower than 20 mT.

Further characteristics and advantages will appear more clearly from thefollowing description of some examples of superconducting cablesaccording to the invention, made—by way of non-limitativeillustration—with reference to the attached drawings.

In the drawings:

FIG. 1 shows a schematic view, in perspective and partial section, of atriphase superconducting cable, according to an embodiment of thisinvention;

FIG. 2 shows a schematic view, in perspective and partial section, of asingle phase superconducting cable, according to a further embodiment ofthis invention;

FIG. 3 shows a further embodiment of a cable according to thisinvention, using low-temperature superconductors;

FIG. 4 shows an electric connection scheme of a single phase cableaccording to the invention with two independent loads; and

FIG. 5 shows a qualitative graph of magnetic field values within coaxialconductors including superconducting material and contained in aconductive element.

With reference to FIG. 1, a triphase superconducting cable 1 accordingto this invention comprises a superconducting core globally indicated by2, comprising a plurality of conductive elements 3, indicated by 3 a, 3b, 3 c for each phase, housed—preferably loosely—within a tubularcontaining shell 9, made e.g. of metal, such as steel, aluminium and thelike.

Each of the conductive elements 3 comprises in turn a couple of coaxialconductors, respectively phase and neutral conductors 4, 5, eachincluding at least one layer of superconducting material.

In the examples shown in the drawings, the superconducting material isincorporated in a plurality of superimposed tapes, wound on respectivetubular supporting elements 6 and (possibly) 7, made of a suitablematerial, for instance formed with a spiral-wound metal tape, or with atube made of plastics or the like.

The coaxial phase conductors 4 and neutral conductors 5 are electricallyinsulated from one another by interposing a layer 8 of dielectricmaterial.

Cable 1 also comprises suitable means to cool the superconducting core 2to a temperature adequately lower than the critical temperature of thechosen superconducting material, which in the cable of FIG. 1 is of theso called “high-temperature” type.

The aforementioned means comprises suitable pumping means, known per seand therefore not shown, supplying a suitable cooling fluid, forinstance liquid nitrogen at a temperature typically of from 65° to 90°K, both in the inside of each of the conductive elements 3 and in theinterstices between such elements and the tubular shell 9.

In order to reduce as much as possible the thermal dissipations towardsthe external environment, the superconducting core 2 is enclosed in acontaining structure or cryostat 10, comprising a thermal insulation,formed for instance by a plurality of superimposed layers, and at leasta protection sheath.

A cryostat known in the art is described, for instance, in an article ofIEEE TRANSACTIONS ON POWER DELIVERY, Vol. 7, nr. 4, October 1992, pp.1745-1753.

More particularly, in the example shown, the cryostat 10 comprises alayer 11 of insulating material, formed, for instance, by severalsurface-metallized tapes (some tens) made of plastics (for instance, apolyester resin), known in the art as “thermal superinsulator”, looselywound, with the possible help of interposed spacers 13. Such tapes arehoused in an annular hollow space 12, delimited by a tubular element 14,in which a vacuum in the order of 10⁻² N/m² is maintained by means ofknown apparatuses.

The tubular element 14 made of metal is capable of providing the annularhollow space 12 with the desired fluid-tight characteristics, and iscovered by an external sheath 15, for instance made of polyethylene.

Preferably, the tubular metal element 14 is formed by a tape bent intubular form and welded longitudinally, made of steel, copper, alumimiumor the like, or by an extruded tube or the like.

If the flexibility requirements of the cable so suggest, element 14 maybe corrugated.

In addition to the described elements, cable traction elements may alsobe present, axially or peripherally located according to theconstruction and use requirements of the same, to ensure the limitationof the mechanical stresses applied to the superconducting elements 3.Such traction elements, not shown, may be formed, according totechniques well known in the art, by peripherally arranged metalreinforcements, for instance by roped steel wires, or by one or moreaxial metal ropes, or by reinforcements made of dielectric material, forinstance aramidic fibers.

According to the invention, several superconducting elements are presentfor each phase, in particular, as shown by way of example in FIG. 1,each phase (a, b, c) comprises two superconducting elements,respectively indicated by the subscripts 1, 2 for each of the threeillustrated superconducting elements 3 a, 3 b, 3 c, so that the currentof each phase is split up among several conductors (two in the exampleshown) shown on FIG. 1 as superconducting elements 3 a ₁ and 3 a ₂, 3 b₁ and 3 b ₂ or 3 c ₁ and 3 c ₂.

FIGS. 2 and 3 schematically show two different embodiments of thisinvention, both of them relating to a monophase cable.

In the following description and in the figures, the components of thecable structurally or functionally equivalent to those previouslydescribed with reference to FIG. 1 will be indicated by the samereference numbers and will be no longer discussed.

In the embodiment of FIG. 2 four superconducting elements 3 ^(I), 3^(II), 3 ^(III), 3 ^(IV), structurally independent and magneticallyuncoupled, are enclosed in the tubular containing shell 9.

In the cable of FIG. 3, phase and neutral coaxial conductors 40, 50 offour elements 30 ^(I), 3 ^(II), 30 ^(III), 30 ^(IV), comprise asuperconducting material made of niobium-titanium alloy, for which thesuperconductivity conditions are reached by cooling the superconductingcore 2 to about 4° K by means of liquid helium.

In this further embodiment, the cryostat comprises, besides a firstlayer of tapes 11, a hollow space 16 in which liquid nitrogen circulatesat 65°-90° K, and a second layer of tapes 17, having a structure similarto the preceding ones.

FIG. 4 schematically shows an example of connection of the fourelements, wherein a monophase generator G is connected to the respectivephase and neutral superconductors 4 and 5 of elements 3 ^(I), 3 ^(II), 3^(III)and, 3 ^(IV). Each of the elements 3 ^(I), 3 ^(II) and 3 ^(III) isconnected to a first load C₁ and element 3 ^(IV) is independentlyconnected to a second load C₂.

With reference to what has been described hereinabove, some examples ofsuperconducting cables according to the invention will be describedhereunder by way of non-limitative illustration.

EXAMPLES 1-3 (Invention)

According to the invention, three high power superconducting cables ofthe monophase type were designed, incorporating respectively 37, 19 and7 conductive elements 3 within the superconducting core 2.

All the cables were designed to be used in d.c. at a voltage of 250 kV(high voltage), using a thickness of the dielectric layer equal to 10mm.

In all the cables the superconducting material used was the mixed oxideknown as BSCCO-2223.

As the cryogenic fluid used in this case is constituted by liquidnitrogen at a temperature of 65 to 90° K, the cables possess thestructure schematically illustrated in FIG. 2, using a cryostat 10having an overall thickness equal to about 10 mm.

The design current was equal to 50 kA.

The design characteristics in d.c. of the cables were:

working magnetic field at the decay threshold of the critical currentdensity, at the temperature of the cryogenic fluid (about 77° K)=20 mT;and

working magnetic field to which corresponds a critical current densityequal to 50% of that with a field ≦20 mT, at the temperature of thecryogenic fluid (about 77° K)=100 mT.

As to d.c. losses, it has been assumed by way of approximation that:

the losses of the conductor were negligible compared with the otherlosses;

the losses in the dielectric were negligible compared with the otherlosses;

the thermal dissipation losses from the cryostat—proportional to thesurface thereof—were expressed by a ratio between the entering thermalpower and the cryostat surface, equal to 3.5 W/m²; and

the efficiency of the cooling plant were expressed by a ratio betweenthe installed power Wi and the extracted thermal power We equal to 10W/W.

Therefore, as a first approximation, it is necessary to install for thecables considered a cooling plant having a power W_(i) equal to 35 W/m².FIG. 5 shows the relationship between the magnetic field and radius of aconductive element of the superconducting cable according to theinvention.

Then for all cables the mean exploitation efficiency of thesuperconductor was evaluated based on the following working hypotheses:

that the magnetic field generated within the superconducting materialhad to increase linearly from a 0 (zero) value on the internal surfaceof each of the phase coaxial conductors 4 (radius R1) and respectivelyon the external surface of the neutral ones 5 (radius R4), up to maximumvalues respectively on the external surface of the phase conductors 4(radius R2) and on the internal surface of the neutral ones 5 (radiusR3), as is schematically shown in FIG. 5, while in the hollow spacebetween the phase and neutral conductors (between radiuses R2 and R3),the field changes according to the already mentioned law${B = {\frac{\mu_{o}I}{2\quad \pi \quad r} \cdot \frac{R_{2}}{r}}},$

wherein r is the radius of the element and I is the current transmittedby conductors 4 and 5; and

the exploitation efficiency of the superconducting material had adecreasing linear trend through the thickness, with threshold valuesequal to 100% on the surface having zero field and up to the thresholdlevel of the field, and equal to the level corresponding to the decayproduced by the maximum working field on the surface having maximumfield, for each of the phase and neutral conductors (in particular 100%was assumed between 0 and 20 mT and 50% at 100 mT).

The structural and functional characteristics of the resulting cablesare summarized in the following table I.

EXAMPLE 4 (Comparison)

In order to compare the cables of the invention with those of the priorart, a cable was designed comprising within the core 2 a single coaxialelement incorporating superconducting material BSCCO-2223 cooled inliquid nitrogen.

The design conditions were the same of preceding examples 1-3, with theadditional working limitation constituted by the fact of keeping a meanexploitation efficiency of the superconducting material equal to 100%.

The structural and functional characteristics of the resulting cablesare summarized in the following table I.

EXAMPLE 5 (Comparison)

Again to compare the cables of the invention with those of the priorart, a cable was designed comprising within the core 2 a single coaxialelement incorporating superconducting material BSCCO-2223 cooled inliquid nitrogen.

The design conditions were the same of the preceding example 4, with theadditional working limitation constituted by the fact of fixing theworking magnetic field to 100 mT.

As a consequence, the mean exploitation efficiency of thesuperconducting material was equal to about 70%.

The structural and functional characteristics of the resulting cable aresummarized in the following table I.

EXAMPLE 5bis (Comparison)

Again to compare the cables of the invention with those of the priorart, a cable was designed comprising within the core 2 a single coaxialelement incorporating superconducting material BSCCO-2223 cooled inliquid nitrogen.

The design conditions were the same of the preceding example 4, withadditional working limitation constituted by the fact of fixing thediameter of the cryostat at a value equal to that of the precedingexample 3 (0.195 m).

As a consequence, the mean exploitation efficiency of thesuperconducting material decreased to a value of about 60%. Therefore,compared with the cable of the invention, it is necessary tointroduce—with the same diameter—a greater quantity of superconductingmaterial with a remarkable increase both of the costs and of thetechnological manufacturing difficulties of the same cable.

The structural and functional characteristics of the resulting cable aresummarized in the following table I.

EXAMPLES 6-8 (Comparison)

In order to compare the cables of the invention with those of the priorart, three cables were designed comprising within the core 2 a singlecoaxial element and incorporating respectively a superconductingmaterial BSCCO-2223 (Example 6) and a niobium-titanium alloy (Examples 7and 8).

Since the cryogenic fluid used was liquid helium at 4° K, the cableshave the structure schematically shown in FIG. 3, using a cryostat 10having an overall thickness equal to about 70 mm.

In these cases, it has been assumed as design data a minimum diameter ofthe single conductive element equal to 0.025 m, to respect theconstruction sizes that maintain the mechanical stresses withinacceptable values.

The d.c. design characteristics were, consequently, a working magneticfield at the temperature of the cryogenic fluid (4° K) of 800 mT, towhich corresponds a current density equal to 100% and 25% of thecritical one, for the Examples 6 and 8 respectively, and a workingmagnetic field of 260 mT at the temperature of the cryogenic fluid (4°K) in Example 7.

As to d.c. losses, it has been assumed, by way of approximation, that:

the losses of the conductor are negligible compared with the otherlosses;

the losses in the dielectric are negligible compared with the otherlosses;

the thermal dissipation losses from the cryostat—proportional to thesurface thereof—are expressed by a ratio between the entering thermalpower and the cryostat surface, equal to 0.5 W/m²; and

the efficiency of the cooling plant is expressed by a ratio between theinstalled power W_(i) and the extracted thermal power W_(e) equal to 300W/w.

Therefore, as a first approximation, it is necessary to install for thecables considered a cooling plant having a power W_(i) equal to 185 W.

Then for all cables the mean exploitation efficiency of thesuperconductor was evaluated based on the criteria illustrated in thepreceding Examples 1-5.

The structural and functional characteristics of the resulting cablesare summarized in the following table I.

EXAMPLES 9-11 (Invention)

According to the invention, three high power superconducting cables weredesigned, incorporating respectively 37, 19 and 7 conductive elementsinside the superconducting core 2.

The design data were the same as for the preceding Examples 1-3, exceptfor the d.c. use voltage, equal in this case to 1 kV (low voltage).

Therefore, a thickness of the dielectric material layer 8 equal to 1 mmwas used.

In all cables, the superconducting material used was the mixed oxideknown as BSCCO-2223.

Since the cryogenic fluid used in this case is liquid nitrogen at atemperature of 77° K, the cables possess the structure schematicallyillustrated in FIG. 1, using a cryostat 10 having an overall thicknessequal to about 10 mm.

Also in this case, the design current was equal to 50 kA.

The structural and functional characteristics of the resulting cablesare summarized in the following table II.

EXAMPLE 12 (Comparison)

In order to compare the cables of the invention with those of the priorart, a cable was designed comprising within the core 2 a single coaxialelement incorporating the superconducting material BSCCO-2223 cooled inliquid nitrogen.

The design conditions were the same of preceding Examples 9-11, with theadditional working limitation constituted by the fact of keeping a meanexploitation efficiency of the superconductor equal to 100%.

The structural and functional characteristics of the resulting cablesare summarized in the following table II.

EXAMPLE 13 (Comparison)

Again in order to compare the cables of the invention with those of theprior art, a cable was designed comprising within the core 2 a singlecoaxial element incorporating the superconducting material BSCCO-2223cooled in liquid nitrogen.

The design conditions were the same of preceding Examples 9-11, with theadditional working limitation constituted by the fact of fixing theworking magnetic field at 100 mT.

As a consequence, the mean exploitation efficiency of thesuperconducting material was equal to 70%.

The structural and functional characteristics of the resulting cablesare summarized in the following table II.

EXAMPLE 13bis (Comparison)

Again in order to compare the cables of the invention with those of theprior art, a cable was designed comprising within the core 2 a singlecoaxial element incorporating the superconducting material BSCCO-2223cooled in liquid nitrogen.

The design conditions were the same of preceding Examples 9-11, with theadditional working limitation constituted by the fact of fixing thediameter of the cryostat at a value equal to the preceding Example 11(0.142 m).

As a consequence, the mean exploitation efficiency of thesuperconducting material dropped to a value of about 50%.

Therefore, compared with the cable of the invention, it is necessary tointroduce—with the same diameter—a greater quantity of superconductingmaterial with a remarkable increase both of the costs and of thetechnological manufacturing difficulties of the same cable.

The structural and functional characteristics of the resulting cablesare summarized in the following table II.

EXAMPLES 14-16 (Comparison)

In order to compare the cables of the invention with those of the priorart, three cables were designed comprising within the core 2 a singlecoaxial element and incorporating respectively a superconductingmaterial BSCCO-2223 (Example 14) and a niobium-titanium alloy (Examples15 and 16).

As the cryogenic fluid used was liquid helium at 4° K, the cables havethe structure schematically shown in FIG. 3, using a cryostat 10 havingan overall thickness equal to about 70 mm.

The design characteristics and the d.c. losses of the cables weredetermined in the same way as that illustrated in Examples 6-9.

The mean exploitation efficiency of the superconducting material wasevaluated based on the criteria illustrated in preceding Examples 1-5.

The structural and functional characteristics of the resulting cablesare summarized in the following table II.

In the following tables I and II, the cooling costs have been indicatedwith reference, respectively, to the cables of Examples 3 and 11, forwhich the size and the costs for cooling the superconducting core 2resulted to have a minimum value, at the loss of a non optimum use ofthe superconducting material, with the ensuing need of using a greaterquantity of the same and with a higher level of electric losses.

With regard to the data reported in tables I and II, it should also benoted that the material BSCCO-2223 works with a 100% efficiency with amagnetic field equal to 800 mT (Examples 6 and 14), and that the NbTialloy has, on the contrary, a 100% efficiency up to a magnetic field ofabout 260 mT (Examples 7 and 15), and equal to 25% at 800 mT (Examples 8and 16).

From what has been described and illustrated hereinabove, it isimmediately evident that the invention allows to couple a transmissionof high current quantities with an optimum exploitation ofhigh-temperature superconducting materials.

All this is achieved by keeping the size of the cables and the coolingcosts at values fully acceptable from a technological point of view.

If the problems and costs associated to a non optimum use of thehigh-temperature superconductor should not be determinant for thepurposes of the specific application, the invention allows all the sameto reduce to a minimum the size of the cable—as shown by Examples 3 and11—facilitating the construction, transport and installation operations,up to values quite comparable with helium-cooled cables of the knownart, which have much higher manufacturing and operational costs.

In particular, it has to be observed that, while a cable according tothe invention—with the same transmitted current—has an overall diameter(cryostat included) lower than 0.3 m, such as to allow, for instance,its winding on a reel, a cable of the known art, using a single coaxialconductive element, would have a diameter greater than 1 meter, if thesuperconducting material were used at a 100% efficiency (magnetic fieldlower than 20 mT).

In the same way, if a 70% efficiency of the superconducting material isaccepted (magnetic field up to 100 mT), a cable according to thisinvention may have a diameter of 0.14 m, while a cable according to theknown art would have a diameter of no less than 0.23 m, with theassociated drawbacks, such as for instance a 60% increase of the coolingcosts.

It must be noted that the subdivision into several superconductingelements does not involve an increase in the overall surface of the sameconductors, and therefore it does not cause any actual increase in thevolume of the insulation used.

According to the invention, furthermore, it is advantageously possibleto:

reduce the size of the cable—with the same exploitation ofsuperconducting material—with ensuing easiness of construction,transport and installation of the cable (compare Example 2 with Example4, and Example 3 with Example 5);

use—compared with the cables of the known art—the same quantity ofelectric insulation with the same quantity of supeconducting material;

limit the size of the thermal insulation layers (cryostat) whichsurround the superconducting core of the cable, with an advantageousreduction in thermal losses (compare Examples 1 and 2 with Example 4,and Example 3 with Example 5);

have magnetically uncoupled conductive elements capable of supplyingdifferent loads;

make flexible, high-efficiency superconducting bus bars; and

use in the best way and therefore reduce the quantity of superconductingmaterial present in the various phase and neutral conductors, with thesame cable diameter and therefore also with the same cooling costs.

It should be noted that, should one wish to make a high voltage cable(250 KV) with a diameter of 0.14 m according to the known art, i.e. witha single element of the coaxial type, a magnetic field of 175 mT wouldbe reached to which corresponds an exploitation efficiency of thesuperconducting material equal to 50%, compared with the 70% obtainableaccording to the invention (see on the matter Examples 3 and 5bis).

In the same way, should one wish to make a low voltage cable (1 KV) witha diameter of 0.2 m according to the known art, i.e. with a singleelement of the coaxial type, a magnetic field of 130 mT would be reachedto which corresponds an exploitation efficiency of the superconductingmaterial equal to 60%, compared with the 70% obtainable according to theinvention (see on the matter Examples 11 and 13bis).

What has been illustrated with reference to cables of the monophasetype, applies also to cables of the triphase type or, more generally,multi-phase, of the type shown in FIG. 1, in which a remarkableadvantage is reached by splitting up the conductive elements of eachphase into several elements, each of which carries a fraction of theglobal current of the phase.

For instance, a triphase cable for supplying 1700 MVA at 20 KV,manufactured with a single conductive element for each phase wouldrequire a diameter on the cryostat of 0.52 m.

According to the present invention, by splitting up each phase into 7phase conductors, the cable would have a diameter on the cryostat of0.43 m, with the same use of the superconducting material.

In the same way, a triphase cable for supplying 35 MVA at 400 V,manufactured with a single conductive element for each phase, wouldrequire a diameter on the cryostat of 0.48 m. According to the presentinvention, by splitting up each phase into 7 phase conductors, the cablewould have a diameter on the cryostat of 0.32 m, with the same use ofthe superconducting material.

With regard to the method of the invention, it has also been observedthat current quantities higher than a predetermined value, generallyequal to at least 5,000 A, may be carried—with the aforementionedadvantages—by splitting up the total current into a number ofmagnetically independent conductors such that the current fractioncarried within each of them is smaller than a threshold value inducing amagnetic field capable of limiting the conductivity of thesupercondutive material used.

Obviously, those skilled in the art may introduce variants andmodifications to the above described invention, in order to satisfyspecific and contingent requirements, variants and modifications whichfall within the scope of protection as is defined in the followingclaims.

TABLE I Example 1 2 3 4 5 5bis 6 7 8 Material BSCCO NbTi Nr. of 37 19 71 1 1 1 1 1 elements per phase Critical 1350 2630 7140 50000 50000 5000050000 50000 50000 current for cond. [A] Working 77 77 77 77 77 77 4 4 4temp. [° K.] Working 20 20 100 20 100 130 800 260 800 magnetic field[mT] Mean 100 100 70 100 70 60 100 100 90 exploitation efficiency of theSC material [%] (Approx.) φ single 0.027 0.053 0.0285 1 0.2 0.15 0.0250.077 0.025 phase conductor [m] φ single 0.057 0.083 0.0585 1.03 0.230.18 0.055 0.107 0.055 element [m] φ cryostat 0.419 0.435 0.195 1.050.25 0.195 0.195 0.247 0.195 [m] Cooling costs 2.1 2.2 1 5.4 1.3 1 5.36.7 5.3

TABLE II Example 9 10 11 12 13 13bis 14 15 16 Material BSCCO NbTi Nr. of37 19 7 1 1 1 1 1 1 elements per phase Critical 1350 2630 7140 5000050000 50000 50000 50000 50000 current for cond. [A] Working 77 77 77 7777 77 4 4 4 temp. [° K.] Working 20 20 100 20 100 175 800 260 800magnetic field [mT] Mean 100 100 70 100 70 50 100 100 90 exploitationefficiency of the SC material [%] (Approx.) φ single 0.027 0.053 0.02851 0.2 0.11 0.025 0.077 0.025 phase conductor [m] φ single 0.039 0.0650.0405 1.012 0.212 0.122 0.037 0.089 0.037 element [m] φ cryostat 0.2930.343 0.142 1.032 0.232 0.142 0.177 0.229 0.177 [m] Cooling costs 2.12.4 1 7.3 1.6 1 6.6 8.6 6.6

We claim:
 1. A high power superconducting cable comprising: asuperconducting core comprising a plurality of magnetically uncoupledconductive elements having opposing ends for connection, respectively,to a source for providing a predetermined current having at least onephase and to at least one load, wherein each of said conductive elementscomprises a couple of phase and neutral conductors, wherein each of theneutral conductors is external and coaxial to a respective one of thephase conductors, wherein each of the phase and neutral conductorsincludes at least a layer of a superconducting material, each of saidphase and neutral conductors being electrically insulated from oneanother by interposition of a dielectric material and each of saidconductive elements being electrically insulated from one another; meansfor cooling said core at a temperature not higher than the criticaltemperature of said superconducting material; and means for electricallyconnecting, at one of the opposing ends, a predetermined number of theconductive elements to each other such that each of the predeterminednumber of the conductive elements carries a fraction of thepredetermined current provided by the source, wherein said predeterminednumber is of a value such that the fraction of the predetermined currentis lower than a value which determines a superficial current density ineach of the predetermined number of the conductive elementscorresponding to a magnetic field capable of generating a conductivityreduction in the superconducting material and wherein thesuperconducting material is distributed among the predetermined numberof conductive elements to provide that the mean exploitation efficiencyof the superconducting material is about 100%.
 2. Superconducting cableaccording to claim 1, characterized in that each of said phase andneutral conductors comprises a plurality of tapes of saidsuperconducting material wound on tubular cylindrical supports. 3.Superconducting cable according to claim 2, characterized in that eachof said phase and neutral conductors comprises a plurality of layers ofsaid superconducting material placed on said tubular cylindricalsupports.
 4. Superconducting cable according to claim 2, characterizedin that said tapes of superconducting material are wound on saidsupports with windup angles of 10° to 60°.
 5. Superconducting cableaccording to claim 1, characterized in that the diameter of the phaseconductor of each of said conductive elements is between 25 and 40 mm.6. Superconducting cable according to claim 1, characterized in thatsaid core is cooled at a temperature from 65° to 90° K. 7.Superconducting cable according to claim 1, characterized in that saidcore is cooled by means of liquid helium at a temperature of about 4° K.8. Superconducting cable according to claim 1, characterized in thatsaid superconducting material has the following formula:Bi_(α)PB_(β)Sr_(γ)Ca_(δ)Cu_(ε)O_(x)  (I) wherein α is a number from 1.4to 2.0; β is a number from 0 to 0.6; γ is a number from 0 to 2.5; δ is anumber from 0 to 2.5; ε is a number from 1.0 to 4.0; and x is thestoichiometric value corresponding to the different oxides present. 9.The superconducting cable of claim 1, wherein said phase conductors ofsaid predetermined number of the conductive elements are electricallyconnected in parallel.
 10. A system for transmitting a current quantitygreater than a predetermined value within a superconducting cablecomprising: a generator having at least one phase; a superconductingcore comprising, for the at least one phase, a plurality of coaxialsuperconductors, wherein each of the superconductors is connected tosaid one phase and includes at least a layer of a superconductingmaterial and wherein each of the superconductors is electricallyinsulated from one another and carries a fraction of the current beingtransmitted; at least one load; and means for electrically connecting apredetermined number of said plurality of the coaxial superconductors toeach other and to said at least one load, wherein said predeterminednumber is of a value such that the fraction of the current is lower thana value which determines a superficial current density in each of thepredetermined number of the superconductors corresponding to a magneticfield capable of generating a conductivity reduction in thesuperconducting material and wherein the superconducting material isdistributed among the respective predetermined number of superconductorsto provide that the mean exploitation efficiency of the superconductingmaterial is about 100%.
 11. A high power superconducting cable forcarrying a predetermined high current and comprising: a plurality ofsuperconducting conductors within a sheath and having opposing ends forconnection, respectively, to a source for providing a predeterminedcurrent having at least one phase and to at least one load, each of saidsuperconducting conductors comprising a phase conductor ofsuperconducting material and a neutral conductor which is a layer of asuperconducting material coaxial with said phase conductor and each ofsaid superconducting conductors being electrically insulated andmagnetically uncoupled from one another; means for cooling saidsuperconducting conductors to a temperature not higher than the criticaltemperature of said superconducting material; and means for electricallyconnecting, at one of the opposing ends, a predetermined number of theconductors to each other such that each of the predetermined number ofsaid conductors carries a fraction of the current provided by thesource, wherein said predetermined number is of a value such that thefraction of the current is lower than a value which determines asuperficial current density in each of the predetermined number of theconductors corresponding to a magnetic field capable of generating aconductivity reduction in the superconducting material and wherein thesuperconducting material is distributed among the respectivepredetermined number of conductive elements to provide that the meanexploitation efficiency of the superconducting material is about 100%.12. A system for transmitting a current quantity greater than apredetermined value within a superconducting cable comprising: agenerator having at least one phase; a high power superconducting cablecomprising: a superconducting core comprising, for the at least onephase, a plurality of magnetically uncoupled conductive elements,wherein each of said conductive elements is connected to said at leastone phase to carry a fraction of the current being transmitted andcomprises a couple of phase and neutral conductors, wherein each of theneutral conductors is external and coaxial to a respective one of thephase conductors, wherein each of the phase and neutral conductorsincludes at least a layer of a superconducting material, wherein saidphase and neutral conductors are electrically insulated from one anotherby interposition of a dielectric material and wherein each of saidconductive elements is electrically insulated from one another; andmeans for cooling said core at a temperature not higher than thecritical temperature of said superconducting material; at least oneload; and means for electrically connecting a predetermined number ofsaid plurality of conductive elements to each other and to said at leastone load, wherein said predetermined number is of a value such that thefraction of the current is lower than a value which determines asuperficial current density in each of the predetermined number of thesuperconductors corresponding to a magnetic field capable of generatinga conductivity reduction in the superconducting material and wherein thesuperconducting material is distributed among said predetermined numberof superconductors to provide that the mean exploitation efficiency ofthe superconducting material is about 100%.
 13. A system fortransmitting a current quantity greater than a predetermined valuecomprising: a generator having at least one phase; a high powersuperconducting cable comprising: a plurality of superconductingconductors within a sheath, wherein each of said superconductingconductors comprises a phase conductor of superconducting material and aneutral conductor which is a layer of a superconducting material coaxialwith said phase conductor, wherein said superconducting conductors areelectrically insulated and magnetically uncoupled from one another andwherein each of said superconducting conductors is connected to said atleast one phase to carry a fraction of the current being transmitted,and means for cooling said core at a temperature not higher than thecritical temperature of said superconducting material; at least oneload; and means for electrically connecting a predetermined number ofsaid plurality of superconducting conductors to each other and to saidat least one load, wherein said predetermined number is of a value suchthat the fraction of the current is lower than a value which determinesa superficial current density in each of the predetermined number of thesuperconductors corresponding to a magnetic field capable of generatinga conductivity reduction in the superconducting material wherein asuperconducting material is distributed in the layers of thesuperconducting material in the respective predetermined number ofsuperconductors to provide that the mean exploitation efficiency of thesuperconducting material is about 100%.